This invention relates to the synthesis of functional poly(meth)acrylates, particularly hyperbranched poly(meth)acrylates, from their corresponding inimers. These inimers and precursor esters are synthesized from halohydrins.
The effect of different architectures on the chemical and physical properties of the polymers have been an area of research for many years, including of poly(meth)acrylates, which are of commercial and academic interest. Varieties of architectures like linear, star, graft, cyclic, dendritic and hyperbranched poly(meth)acrylates have been synthesized and their physical properties are under investigation.
Halohydrins are typically synthesized by either direct hydrohalogention of the corresponding olefin, or by the first converting the olefin to an epoxide, followed by reaction with HX (HCl/HBr). Ring-opening of glycidic esters with HCl and HBr generates the wrong regioisomer (Kuroyan et. al. and Talasbaeva et al.), with —OH alpha to the ester. Hydrobromination of methacrylates produces a mixture of regioisomers (Farook et. al.). Hydrobromination of acrylates also produces primarily the wrong regioisomer (Slicker et. al.) in low yield due to the formation of a large amount of dibromide as side product (Bell et. al.), although the products were initially assumed to be 2-bromo-3-hyroxypropionate (Mattocks et. al.); the amount of dibromide can be reduced by adding AgNO3 to precipitate AgBr out of the reaction mixture (Leibman et. al.). We have found a much cleaner reaction is to convert the amine group of serine and its ester to a halogen group by diazotization in the presence of KX (Br/Cl) as shown in Scheme 5 (Larchevêque et. al. and Shimohigashi et. al.). The short alkyl esters of serine are either commercially available as HCl salts or are easily synthesized by acid-catalyzed esterification using the desired alcohol as solvent.
Chemically similar polymers having different molecular architecture can exhibit various interesting properties that are different than the polymers of conventional architectures (like linear and branched, cross-linked polymers). Most importantly and distinctly, shear thinning behavior and lower viscosity of these polymers give processing advantages compared to the linear counterparts. This new class of architecture mainly consists of dendrimers and hyperbranched polymers. In contrast to dendrimers, which have uniform distribution of branches in three dimensions, hyperbranched polymers are characterized by random and non-uniform branching. It has been suggested in the reported literatures that dendrimers can successfully be employed in certain applications to achieve improved properties, especially processing properties. Due to lack of entanglements of the chains, the viscosity of these polymers is lower than that of linear polymers. These polymers also have reactive end groups that can be modified and used advantageously in coating and additive applications. Dendrimers are monodisperse (typically have polydispersity 1.02 or less) (reference: U.S. Pat. No. 6,812,298 B2) and synthesized with controlled step-growth reactions with tedious protection-deprotection strategies and purification. In contrast, hyperbranched polymers are made from one-step, one-pot reactions and are polydisperse. This facilitates the synthesis of a large amount of polymers with higher yield at comparatively lower cost. Due to its imperfect branching and higher polydispersity, the properties of hyperbranched polymers lie between those of dendrimers and linear polymers. This wide window of properties between these of the two extreme architectures makes hyperbranched polymers a potential competitor superior to dendrimers in certain applications.
Until now, the synthetic techniques used to prepare hyperbranched polymers could be divided into two major categories. The first category contains techniques of the single-monomer methodology in which hyperbranched polymers are synthesized by the polymerization of an ABn, monomer. This method also includes self condensing vinyl polymerization (SCVP). The other category contains methods of the double-monomer methodology in which two types of monomers or a monomer pair generates hyperbranched polymers. (C. Guo, D. yan; Prog. Polym. Sci. 29 (2004), 183-275.)
Fréchet and co-workers proposed SCVP in which a vinyl monomer can be self-polymerized if it has a pendant group that can be transformed into an initiating site by the action of external stimulus (Fréchet et. al., Science, 1995, 269, 1080-1083). Since there are two polymerizing growth sites (vinylic and initiating) and the activities of these sites may differ with each other, the degree of branching (DB) (which is defined as the number of branch units present in the architecture with respect to the total number of different structural units) will have different values for the different systems and/or at different conditions below a theoretical maximum value (DB=0.465); (this value was obtained by theoretical calculations done by Müller et. al., Macromolecules 1997, 30, 7024-7033.) detailed theoretical investigations for the hyperbranched polymers have been done by Müller and co-workers. Hyperbranched polymers obtained by SCVP generally have broad molecular weight distribution and any side reaction may lead to cross-linking during synthesis of these polymers. Living polymerizations like atom transfer radical polymerization and group transfer polymerization (GTP) are employed to better control the architecture of these polymers. Numerous styrene and (meth)acrylate based monomers and inimers have been synthesized to produce hyperbranched polymers using the concept of SCVP and living radical polymerization.
Linear poly(meth)acrylates with free ester side chains of different functional groups can be synthesized but hyperbranched structure using SCVP of inimer having different functional groups have not been synthesized yet. As an example: although numerous dendrimers and hyperbranched polyacrylates (Busson et. al., Sunder et. al., Peng et. al., Percec and Kricheldorf et. al.) have been synthesized with the mesogens (compounds that under suitable conditions of temperature, pressure, and concentration can exhibit a liquid crystal phase) are attached only at their periphery, or within the main chain of the polymer, none have been synthesized with the mesogens attached as a side chain throughout the branched structure.
In addition, the hyperbranched polyacrylates and poly(meth)acrylates synthesized by homopolymerization of an inimer were not analogs of linear poly(meth)acrylates. In contrast, the first hyperbranched polystyrene (Hawker et. al.) produced by SCVP (Fréchet et al.) of an inimer by a radical mechanism produced a hyperbranched polymer that is fairly analogous to linear polystyrene (but with an extra —CH2O—) (Scheme 1).
Subsequently synthesized “hyperbranched polystyrenes” (Gaynor et. al., Weimer et. al., Ishizu et. al.) such as the second example in Scheme 1, incorporate the aromatic ring within the main chain of the polymer and therefore more analogous to polymers produced by step polymerizations; in addition, all free aromatic groups not incorporated into branches are functionalized with an initiator fragment. Similarly, all of the hyperbranched poly(meth)acrylates (Matyjaszewski et. al. and Yoo. et. al.) synthesized to date by SCVP incorporate the alkyl ester into the polymer backbone upon branching (Scheme 2), and leave an alkyl ester side chain functionalized with an initiator fragment at incomplete branching. These polymers are therefore not analogs of linear poly(meth)acrylates, whose properties could be compared to determine architectural effects. They are also not analogs of the branched poly(meth)acrylates produced in conventional radical polymerizations in which branching occurs by chain transfer at a site along the polymer backbone, rather than at the ester side chains.
This method synthesizes the first hyperbranched analogs of the linear poly(meth)acrylates from the corresponding inimers based on a halohydrin (bromoydrin/chlorohydrin) intermediate. The detailed description of which is given in the subsequent sections.